1. Field of the Invention
The present invention relates generally to variable frequency oscillators and in particular to variable frequency oscillators which must maintain a particular frequency of oscillation despite temperature variations. The present invention further relates to oscillators that must provide sufficient frequency pull range to respond to both frequency compensation circuitry and to frequency correction signals due to normal circuit operations.
2. Description of the Related Art
Voltage or current controlled variable frequency oscillators are frequently used in precision clock generators for digital systems. Such variable oscillators have a frequency control input that permits the oscillator's frequency to be modified in accordance with a frequency control signal. However, the oscillator's operating range for responding to the frequency control signal is limited, if not by the particular architecture of the oscillator then by the physical limitations of the circuit itself. For example, a voltage controlled oscillator, VCO, has a frequency control input whose control voltage range is ultimately limited by the supply voltages. Furthermore, the operating range of the frequency control signal is shared by all mechanisms that require frequency control of the oscillator.
For example, in addition to normal frequency selection input control, a precision oscillator may need to compensate for normal circuit variations, such as voltage fluctuations. This is typically done via a feedback path that provides automatic frequency control, AFC, by monitoring the output frequency and applying an appropriate frequency compensation signal to offset any observed frequency variations.
Furthermore, precision oscillators typically also require temperature compensation frequency control to compensate for temperature variations. In these systems, a temperature sensor monitors temperature variations and applies a frequency compensation signal to the variable oscillator so as to offset the natural frequency drift effects of temperature variations on the oscillator.
In the above mentioned frequency compensation techniques, it is important that the oscillator's frequency control input range (i.e. the control voltage range in a VCO) be large enough to accommodate the added frequency control requirements of the AFC and/or temperature compensation control mechanisms. Thus, oscillators that provide temperature compensation control require that their total available frequency compensation input range include sufficient frequency pull range to respond to the temperature compensation signal in addition to the required functional control range for compensating for normal system frequency variations.
However, since the total control range is limited by the supply voltages, the needed temperature compensation requirements necessarily reduces the functional frequency pull range of the oscillator. This imposes more stringent requirements on the functional behavior of the oscillator.
Thus, a precision oscillator would sometimes have either an automatic frequency control, AFC, mechanism with no temperature compensation circuitry; in which case the oscillator would necessarily be constructed of expensive temperature insensitive components or be limited to more stringent ambient operating conditions. Alternatively, the oscillator would have a temperature compensation circuit, but limited frequency compensation control for normal circuit variations; in which case the oscillator would need to be constructed to very stringent normal operating requirements.
An added reason for the limited use of both temperature and circuit fluctuation frequency control is that since an AFC functions by observing the output frequency and attempting to correct for any observed frequency variations, an AFC would tend to respond to both non-intended changes in frequency due to circuit fluctuations and to purposely applied frequency compensation correction from a temperature compensation circuit. In effect, the AFC would tend to reduce the effectiveness of the temperature compensation.
One approach to addressing this issue is shown in U.S. Pat. No. 5,977,839 to Tsumura in which an oscillator having both temperature compensation control and AFC control is shown. In this case, Tsumura adds the temperature compensation signal and the AFC control signal to form one composite control signal that is applied to a frequency control input of an oscillator. To reduce the amount of interference between the temperature compensation control operation and the AFC operation, Tsumura uses a system in which the temperature compensation circuit and the AFC control circuit take turns separately observing and modifying the oscillator's operation. That is, Tsumura's system first holds the output of the AFC circuit constant while the temperature compensation circuit is operating, and then holds the output of the temperature compensation circuit constant while the AFC circuit is operating. As a result, however, Tsumura suggests converting the control signals from the temperature compensation circuit and from the AFC circuit into digital form in order to hold the respective control signals in digital latches, and to more easily sum the control signals from the temperature compensation and AFC circuits. This, of course, also requires a digital-to-analog converter in order to apply an analog representation of the composite (i.e. summed) digital control signals to the frequency control input of the oscillator.
Tsumura's approach offers only a partial solution, however, since the temperature compensation circuit and the AFC circuit cannot provide independent, and concurrent, control over the oscillator (i.e. one is halted while the other is in operation), and further does not address the issue of reduced frequency pull range due to the need to accommodate the corrective pulling action of both the temperature compensation circuit and the AFC circuit.
A method of improving the precession of an oscillator so as to reduce the need for much tuning later when in normal use, is to fine tune the oscillator's operating conditions at the manufacturing stage prior to it being shipped to a customer. U.S. Pat. No. 6,323,739 to Andrews shows a system wherein a reference signal, A/D converter, ROM, and D/A converter are used to fine tune an oscillator while it is still at the manufacturing stage. That is, the oscillator is activated and its performance is fine tuned at the manufacturing stage using the reference signal to select appropriate bias levels that pull the oscillator's frequency until a desired target frequency is achieved. The appropriate bias levels are stored in the ROM, and thus optimal bias conditions for high precision operation are fixed into the oscillator prior to it being shipped to a customer.
Andrews uses load pulling to alter the frequency of his oscillator at the manufacturing stage. Load pulling typically provides only a very small tuning range, and consequently Andrews uses this load pulling technique only at the manufacturing stage to fine tune the oscillator to the target frequency, and does not rely on load pulling for normal use by customers. This is substantiated by Andrews's use of a more traditional tuning technique in his complete system, where oscillation tuning is provided by summing a functional tuning voltage and a correction voltage and applying the sum to a single input of this oscillator. This permits Andrews to obtain a larger correction range than would be possible with load pulling, but still suffers from the limitation of a reduced functional tuning range since the functional tuning voltage and correction voltage are still being summed, as explained above. Thus, although Andrews's approach may reduce the amount of tuning required later when in normal use by a customer, it does not address the issue of how to provide multiple frequency control mechanisms with sufficient control range for each, given the limited and finite tuning range of an oscillator's frequency control input. Basically, Andrews does not show how, for example, sufficient temperature compensation control and function frequency control may be achieved within the limited range of the oscillator's frequency control input.
What is needed is a means of providing temperature compensation control without reducing the frequency tuning range (i.e. voltage range for frequency control) of the oscillator available for normal functional frequency control.